Spinal cord injury was once deemed irreversible and untreatable but recent repair approaches have shown great promise in facilitating axon regeneration across the lesion site and functional recovery. One promising repair strategy is the transplantation of growth-promoting olfactory ensheathing glia (OEG). Our preliminary data and all proposed studies combine the therapies of OEG transplantation with 6 months of treadmill step training to promote axon regeneration following complete spinal cord transection in adult rats. Initial data suggest that these two repair strategies result in functional recovery of hindlimb stepping and substantial regeneration of serotonergic (5-HT) and noradrenergic (NA) axons across the lesion and into the caudal stump. The proposed experiments will determine the potential of these repair strategies, alone or in combination, and examine the mechanisms by which motor function improves. Specific aim 1 will determine if the recovery of hindlimb locomotion following both treatments is due to the regeneration of 5-HT and/or NA axons across the lesion. By pharmacologically blocking 5-HT and/or NA receptors linked to locomotion, the contribution of each of these neurotransmitter systems will be tested. Light and electron microscopy will be used to determine the number and length of the 5-HT and NA axons that cross the lesion site and if they form structurally identifiable synaptic contacts on neurons in the caudal stump. Specific aim 2 will use electrophysiological and pharmacological methods to compare functional connectivity between supraspinal neurons and the lumbosacral motor networks in rats with and without OEG injections and step training. It will also determine if these functional connections use serotonin or noradrenaline for neurotransmission. Specific aim 3 will use retrograde tracing techniques to identify the supraspinal neurons that regenerate their axons across the lesion site and determine whether these retrogradely labeled neurons are serotonergic and/or noradrenergic. Our long term goal is to regain voluntary control of hindlimb stepping after a complete midthoracic spinal cord injury in an adult animal.